JUnit is a unit testing framework for Java, created by Erich Gamma and Kent Beck.
The Apache Commons IO library contains utility classes, stream implementations, file filters, file comparators, endian transformation classes, and much more.
Java.net - The Source for Java Technology Collaboration
Apache Commons Lang, a package of Java utility classes for the classes that are in java.lang's hierarchy, or are considered to be so standard as to justify existence in java.lang.
General data-binding functionality for Jackson: works on core streaming API
Guava is a suite of core and expanded libraries that include utility classes, google's collections, io classes, and much much more. Guava has only one code dependency - javax.annotation, per the JSR-305 spec.
Apache Commons Logging is a thin adapter allowing configurable bridging to other, well known logging systems.
Commons Lang, a package of Java utility classes for the
classes that are in java.lang's hierarchy, or are considered to be so
standard as to justify existence in java.lang.
The Apache Commons Codec package contains simple encoder and decoders for various formats such as Base64 and Hexadecimal. In addition to these widely used encoders and decoders, the codec package also maintains a collection of phonetic encoding utilities.
Hibernate's Bean Validation (JSR-303) reference implementation.
OSGi Core Release 6, Interfaces and Classes for use in compiling bundles.
Spice up your java: Automatic Resource Management, automatic generation of getters, setters, equals, hashCode and toString, and more!
The Apache Commons FileUpload component provides a simple yet flexible means of adding support for multipart file upload functionality to servlets and web applications.
Core Jackson processing abstractions (aka Streaming API), implementation for JSON
Field and method binding for Android views.
Rich and fluent assertions for testing
The MyBatis SQL mapper framework makes it easier to use a relational database with object-oriented applications. MyBatis couples objects with stored procedures or SQL statements using a XML descriptor or annotations. Simplicity is the biggest advantage of the MyBatis data mapper over object relational mapping tools.
Commons Database Connection Pooling
Guice is a lightweight dependency injection framework for Java 6 and above
Data Mapper package is a high-performance data binding package built on Jackson JSON processor
A powerful image downloading and caching library for Android